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New combustion synthesis route to TiB2-Al2O3
Pergamon
Materials Research Bulletin 36 (2001) 1487–1493
New combustion synthesis route to TiB2-Al2O3 Ross H. Plovnick*, Elizabeth A. Richards 3M Company, 3M Center, St. Paul, MN 55144-1000, USA (Refereed) Received 4 January 2001; accepted 21 February 2001
Abstract A new aluminothermic combustion synthesis route to TiB2-Al2O3 composite was found, using the reactants 2Al2O3䡠B2O3 (Al4B2O9), TiO2, and Al. Microwave-driven combustion of these reactants in inert atmosphere yielded TiB2 ⫹ Al2O3 as the only products detected by X-ray diffraction analysis. The aluminoborate reactant is not moisture-sensitive or water-soluble, unlike boron-containing oxide reactants used previously. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramics; A. Composites; B. Chemical synthesis
1. Introduction Titanium diboride, as Hoke et al. [1] have noted, has an attractive combination of high Vickers hardness (15–36 GPa), good fracture toughness (6 – 8 MPa䡠m1/2) and electrical conductivity (9 –15 ⍀䡠cm at 25°C), excellent chemical resistance to molten nonferrous metals, and relatively low specific gravity (4.5 g/cm3). This makes TiB2 and its composites such as TiB2-Al2O3 useful in a variety of applications including cutting tools, wear-resistant substrates, and lightweight armor. Combustion synthesis has been shown to be an effective way to make composites such as TiB2-Al2O3 [2] and TiB2-SiC [1]. A number of potential routes to combustion synthesis of TiB2/Al2O3 composite phases use aluminothermic reduction of oxides of titanium and boron. These typically involve a moisture-sensitive or water-soluble boron oxide reactant [2,3] or
* Corresponding author. Fax: ⫹1-651-575-1942. E-mail address: [email protected] (R.H. Plovnick). 0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 0 6 2 7 - 4
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R.H. Plovnick, E.A. Richards / Materials Research Bulletin 36 (2001) 1487–1493
the expensive elemental B [2], or yield water as an undesirable high-temperature by-product, for example: 1) 2) 3) 4)
3TiO2 3TiO2 3TiO2 3TiO2
⫹ ⫹ ⫹ ⫹
3B2O3 ⫹ 10 Al 3 3TiB2 ⫹ 5Al2O3 6H3BO3 ⫹ 10Al 3 3TiB2 ⫹ 5Al2O3 ⫹ 9H2O 6HBO2 ⫹ 10Al 3 3TiB2 ⫹ 5Al2O3 ⫹ 3H2O 4Al ⫹ 6B 3 3TiB2 ⫹ 2Al2O3
Consideration of alternative chemistries to avoid the need for moisture-sensitive reactants and by-product water suggested this potential reaction: 5) “3TiO2 ⫹ 3(2Al2O3䡠B2O3) ⫹10Al33TiB2 ⫹ 11Al2O3” Although the compound 2Al2O3䡠B2O3 is known [4] and its stability and water-resistance documented [5], the aluminothermic reaction equation 5 does not seem to have been previously reported. The present investigation was carried out to determine whether reaction 5 does occur and proceed to completion as written. If so, this could be a useful alternative preparative technique to previously reported routes to TiB2-Al2O3 composite materials based on combustion synthesis.
2. Experimental 2.1. Preparation of 2Al2O3䡠B2O3 powder An aluminum oxide precursor (approximately 10w% Al2O3) was prepared by dissolving aluminum powder in a mixture of carboxylic acids (aluminum: (total carboxylate) ⫽ 1:2; formate:acetate ⫽ 1:1) [6]. A mixture with an Al/B molar ratio of 2:1 was prepared by dissolving H3BO3 in the aluminum carboxylate at 90oC. The mixture was poured into Pyrex威 baking dishes and oven-dried at 100oC. The dried material was crushed with a disk pulverizer (Type UA, Braun Corp, Los Angeles, Calif.) and sieved to – 600 m. Crushed granules were calcined in shallow mullite boats, heated at 2.5oC/min to 900oC and held at temperature for 0.5 h. X-ray diffraction analysis identified 2Al2O3䡠B2O3. 2.2. Preparation of 3TiO2 ⫹ 3(2Al2O3䡠B2O3) ⫹10Al green bodies The starting point was a 150g batch of anatase-form TiO2 (Kemira, Inc., Savannah, Ga., Unitane 0 –110SP), 2Al2O3䡠B2O3, and Al (Alfa Aesar, Ward Hill, Mass., -325 mesh, 99.5% metals basis) powders in relative amounts according to equation 5. The TiO2 and 2Al2O3䡠B2 O3 powders were dry-mixed in a plastic jar, then attritor-milled in deionized water for 2 h with 5-mm-diameter MgO-stabilized ZrO2 milling media. The milled slurry was screened away from the milling media, air-dried, then oven-dried at 100°C. The dried product was crushed and ground with a porcelain mortar and pestle to pass through a #100 screen (150 m openings), dry-mixed with the Al powder, slurried with deionized water and 0.15 g of Darvan 821A ammonium polyacrylate dispersing agent (R.T. Vanderbilt Co., Norwalk, Conn.), and mixed 4 h with a Jiffy威 Mixer, Model LM (Jiffy Mixer Co., Inc., Riverside,
R.H. Plovnick, E.A. Richards / Materials Research Bulletin 36 (2001) 1487–1493
1489
Fig. 1. Lower-magnification SEM photograph of microwave-combusted mudcracked piece.
Calif.). The thick slurry was poured into Pyrex威 Petri dishes, air-dried, and oven-dried at 120°C. The dried, mudcracked pieces were uniformly gray in color, with no visible bubbles or holes. 2.3. Combustion synthesis of TiB2-Al2O3 A 30 –35g quantity of dried, mudcracked 3TiO2 ⫹ 3(2Al2O3䡠B2O3) ⫹10Al pieces or pellets (⬃10 –20g/pellet, 28.5 mm in diameter x 10 –20 mm thick) uniaxially pressed (⬃100 MPa) from powdered, mudcracked pieces was placed in a ZAL-45AA fibrous alumina (Zircar Products, Inc., Florida, N.Y.) box within an ALC (Zircar) fibrous alumina cylinder. This assembly was positioned inside a quartz bell-jar atmosphere chamber of a Model 10 –1300 1300-watt benchtop microwave furnace (Microwave Materials Technologies, Inc., Knoxville, Tenn.). Temperature was sensed and controlled by a grounded and shielded Mo-sheathed Type C thermocouple in an alumina protective sleeve. After a series of evacuations with a direct drive pump and backfills with helium, the system was maintained in a helium atmosphere. Microwave power was applied to trigger ignition and self-propagating combustion of the sample, as noted from a bright flash and rapid heatup of the sensing thermocouple. Because the thermocouple tip was intentionally isolated from direct contact with the sample in order to protect the thermocouple, only a relative measure of the temperature during ignition could be obtained. After ignition and combustion, full micro-
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Fig. 2. Higher-magnification SEM photograph of microwave-combusted mudcracked piece.
wave power was applied for an additional 20 min. The system was then allowed to cool to room temperature in helium. 2.4. Analysis of reaction products Combusted samples were examined by X-ray diffraction analysis to identify crystalline phases. Data were collected using a Philips vertical diffractometer, copper K␣ radiation, and proportional detector registry of the scattered radiation. The level of each phase was estimated by use of relative intensity values determined from the strongest diffraction peak of each phase. An attempt was made to determine the average crystallite size from the X-ray diffraction analytical data, i.e. the peak intensities at half-width. Scanning electron microscopic (SEM) analysis was carried out on broken pieces of combusted mudcracked pieces and uniaxially pressed pellets.
3. Results and discussion Microwave ignition of the TiO2/2Al2O3䡠B2O3/Al samples occurred at temperatures in the neighborhood of 700⫺750°C, as estimated from the control/reference thermocouple. As noted above, temperature could only be estimated as a result of protective insulation surrounding the thermocouple. Maximum thermocouple temperatures observed approached
R.H. Plovnick, E.A. Richards / Materials Research Bulletin 36 (2001) 1487–1493
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Fig. 3. Lower-magnification SEM photograph of microwave-combusted uniaxially pressed pellet.
1475°C; again, these are only approximations of the actual temperature at the sample surface. It was not possible to take optical pyrometer readings with the available microwave furnace and sample containment system. The microwave-combusted pieces and pellets maintained their shape and weight, did not stick to one another, and were grayish-black vs. whitish-gray prior to combustion. The individual pieces and pellets had good qualitative cohesive strength and sufficient hardness to easily scratch glass deeply. Qualitative testing with a milliohmeter probe indicated the pieces and pellets to have low resistivity, with some surface contact resistance. The microwave-combusted pellets generally cracked into 2–3 pieces with relatively smooth surfaces. Under microscope at 40X, the interior exposed surfaces of cracked pellets appeared crystalline with fine porosity. X-ray diffraction analysis of microwave-combusted pieces (two runs) and pellets (three runs) indicated only ␣-Al2O3 and TiB2. The relative amount of each phase present as estimated by use of relative intensity values determined from the strongest diffraction peak of each phase was ⬃100 Al2O3: 60 TiB2. The attempt to determine the average crystallite size from the peak intensities at half-width showed that the crystallite size of both Al2O3 and TiB2 were ⬎1500 Å, the upper limit of crystallite size which could be determined by this method. Representative SEM analytical photographs are shown in Figures 1– 4 for a combusted mudcracked piece and uniaxially pressed pellet. There are some regions which appear relatively dense, especially in the pellets. The residual porosity is nonuniform, as would be expected from the preparative method used. This investigation has uncovered a new aluminothermic combustion synthesis route to
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Fig. 4. Higher-magnification SEM photograph of microwave-combusted uniaxially pressed pellet.
TiB2-Al2O3 composite, using the reactants 2Al2O3䡠B2O3 (Al4B2O9), TiO2, and Al. Microwave-driven combustion was used for convenience, as it is known to be a useful technique for controllably igniting laboratory-scale aluminothermic reactions [7]. It should, however, be possible to duplicate the synthesis with other known methods of triggering combustion, e.g. resistance heating [8] and many of the specialized methods reviewed by Wang et al. [9] including electrical current, laser beam, and mechanical impact. A major limitation of combustion synthesis is that relatively high levels of porosity, up to 50%, are retained in the product [10]. The SEM photographs in Figs. 1– 4 show this to be the case for the present reaction scheme. In applications where dense final bodies are needed, the products from the present reactant scheme lend themselves to crushing to fine powder followed by a subsequent hot-pressing step. Alternatively, it may be possible to run the combustion synthesis under high pressure, as has been reported for other exothermic chemical reactions [11]. The fact that 2Al2O3䡠B2O3 is not moisture-sensitive or watersoluble, unlike boron-containing oxide reactants used previously to synthesize TiB2-Al2O3, could make use of 2Al2O3䡠B2O3 of particular advantage in high pressure synthesis and in more conventional approaches.
Acknowledgments We thank Brian T. Lynch for the X-ray diffraction analyses, and Chris J. Goodbrake for the scanning electron microscopic analyses.
R.H. Plovnick, E.A. Richards / Materials Research Bulletin 36 (2001) 1487–1493
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References [1] D.A. Hoke, D.K. Kim, J.C. LaSalvia, M.A. Meyers, Combustion synthesis/dynamic densification of a TiB2-SiC composite, J. Am. Ceram. Soc. 79 (1) (1996) 177–182. [2] A. Chrysanthou, D. Cheung, A. Jha, Self-propagating high temperature synthesis of Al2O3-TiB2 ceramics, Conf. on Ceramics - Charting the Future, Proc. 8th CIMTEC, Florence, 28 June– 4 July 1994 Techna Srl, 1973–1980 (1995). [3] K.V. Logan, J.D. Walton, TiB2 formation using thermite ignition, Ceram. Eng. Sci. Proc. 5 (7– 8) (1984) 712–738. [4] P.J.M. Gielisse, W.R. Foster, The system Al2O3-B2O3, Nature 195 (1962) 69 –70. [5] T. Kitamura, K. Sakane, H. Wada, H. Hata, H. Kanbara, Process for preparing aluminum borate whiskers, U.S. Pat No. 4,925,641, 1990. [6] G.W. Warner, Aluminum carboxylates, in: Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed, Vol. 2, Wiley, New York, 1978, pp. 202–209. [7] I. Ahmad, R. Dalton, D. Clark, Unique application of microwave energy to the processing of ceramic materials, J. Microwave Power Electromagn. Energy 26 (3) (1991) 128 –138. [8] J.D. Walton, Jr., N.E. Poulos, Cermets from thermite reactions, J. Am. Ceram. Soc. 42 (1) (1959) 40 – 49. [9] L.L. Wang, Z.A. Munir, Y.M. Maximov, Review [of] thermite reactions: their utilization in the synthesis and processing of materials, J. Mater Sci. 28 (1993) 3693–3708. [10] H.J. Feng, J.J. Moore, D.G. Wirth, Combustion synthesis of dense ceramic-metal composites, Ceram. Trans. 38 (1993) 103–114. [11] R.M. Marin-Ayral, J.C. Tedenac, M. Bockowski, M.C. Dumez, SHS experiments under high pressure, Ann. Chim. Fr. 20 (1995) 169 –180.
Materials Research Bulletin 36 (2001) 1487–1493
New combustion synthesis route to TiB2-Al2O3 Ross H. Plovnick*, Elizabeth A. Richards 3M Company, 3M Center, St. Paul, MN 55144-1000, USA (Refereed) Received 4 January 2001; accepted 21 February 2001
Abstract A new aluminothermic combustion synthesis route to TiB2-Al2O3 composite was found, using the reactants 2Al2O3䡠B2O3 (Al4B2O9), TiO2, and Al. Microwave-driven combustion of these reactants in inert atmosphere yielded TiB2 ⫹ Al2O3 as the only products detected by X-ray diffraction analysis. The aluminoborate reactant is not moisture-sensitive or water-soluble, unlike boron-containing oxide reactants used previously. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramics; A. Composites; B. Chemical synthesis
1. Introduction Titanium diboride, as Hoke et al. [1] have noted, has an attractive combination of high Vickers hardness (15–36 GPa), good fracture toughness (6 – 8 MPa䡠m1/2) and electrical conductivity (9 –15 ⍀䡠cm at 25°C), excellent chemical resistance to molten nonferrous metals, and relatively low specific gravity (4.5 g/cm3). This makes TiB2 and its composites such as TiB2-Al2O3 useful in a variety of applications including cutting tools, wear-resistant substrates, and lightweight armor. Combustion synthesis has been shown to be an effective way to make composites such as TiB2-Al2O3 [2] and TiB2-SiC [1]. A number of potential routes to combustion synthesis of TiB2/Al2O3 composite phases use aluminothermic reduction of oxides of titanium and boron. These typically involve a moisture-sensitive or water-soluble boron oxide reactant [2,3] or
* Corresponding author. Fax: ⫹1-651-575-1942. E-mail address: [email protected] (R.H. Plovnick). 0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 0 6 2 7 - 4
1488
R.H. Plovnick, E.A. Richards / Materials Research Bulletin 36 (2001) 1487–1493
the expensive elemental B [2], or yield water as an undesirable high-temperature by-product, for example: 1) 2) 3) 4)
3TiO2 3TiO2 3TiO2 3TiO2
⫹ ⫹ ⫹ ⫹
3B2O3 ⫹ 10 Al 3 3TiB2 ⫹ 5Al2O3 6H3BO3 ⫹ 10Al 3 3TiB2 ⫹ 5Al2O3 ⫹ 9H2O 6HBO2 ⫹ 10Al 3 3TiB2 ⫹ 5Al2O3 ⫹ 3H2O 4Al ⫹ 6B 3 3TiB2 ⫹ 2Al2O3
Consideration of alternative chemistries to avoid the need for moisture-sensitive reactants and by-product water suggested this potential reaction: 5) “3TiO2 ⫹ 3(2Al2O3䡠B2O3) ⫹10Al33TiB2 ⫹ 11Al2O3” Although the compound 2Al2O3䡠B2O3 is known [4] and its stability and water-resistance documented [5], the aluminothermic reaction equation 5 does not seem to have been previously reported. The present investigation was carried out to determine whether reaction 5 does occur and proceed to completion as written. If so, this could be a useful alternative preparative technique to previously reported routes to TiB2-Al2O3 composite materials based on combustion synthesis.
2. Experimental 2.1. Preparation of 2Al2O3䡠B2O3 powder An aluminum oxide precursor (approximately 10w% Al2O3) was prepared by dissolving aluminum powder in a mixture of carboxylic acids (aluminum: (total carboxylate) ⫽ 1:2; formate:acetate ⫽ 1:1) [6]. A mixture with an Al/B molar ratio of 2:1 was prepared by dissolving H3BO3 in the aluminum carboxylate at 90oC. The mixture was poured into Pyrex威 baking dishes and oven-dried at 100oC. The dried material was crushed with a disk pulverizer (Type UA, Braun Corp, Los Angeles, Calif.) and sieved to – 600 m. Crushed granules were calcined in shallow mullite boats, heated at 2.5oC/min to 900oC and held at temperature for 0.5 h. X-ray diffraction analysis identified 2Al2O3䡠B2O3. 2.2. Preparation of 3TiO2 ⫹ 3(2Al2O3䡠B2O3) ⫹10Al green bodies The starting point was a 150g batch of anatase-form TiO2 (Kemira, Inc., Savannah, Ga., Unitane 0 –110SP), 2Al2O3䡠B2O3, and Al (Alfa Aesar, Ward Hill, Mass., -325 mesh, 99.5% metals basis) powders in relative amounts according to equation 5. The TiO2 and 2Al2O3䡠B2 O3 powders were dry-mixed in a plastic jar, then attritor-milled in deionized water for 2 h with 5-mm-diameter MgO-stabilized ZrO2 milling media. The milled slurry was screened away from the milling media, air-dried, then oven-dried at 100°C. The dried product was crushed and ground with a porcelain mortar and pestle to pass through a #100 screen (150 m openings), dry-mixed with the Al powder, slurried with deionized water and 0.15 g of Darvan 821A ammonium polyacrylate dispersing agent (R.T. Vanderbilt Co., Norwalk, Conn.), and mixed 4 h with a Jiffy威 Mixer, Model LM (Jiffy Mixer Co., Inc., Riverside,
R.H. Plovnick, E.A. Richards / Materials Research Bulletin 36 (2001) 1487–1493
1489
Fig. 1. Lower-magnification SEM photograph of microwave-combusted mudcracked piece.
Calif.). The thick slurry was poured into Pyrex威 Petri dishes, air-dried, and oven-dried at 120°C. The dried, mudcracked pieces were uniformly gray in color, with no visible bubbles or holes. 2.3. Combustion synthesis of TiB2-Al2O3 A 30 –35g quantity of dried, mudcracked 3TiO2 ⫹ 3(2Al2O3䡠B2O3) ⫹10Al pieces or pellets (⬃10 –20g/pellet, 28.5 mm in diameter x 10 –20 mm thick) uniaxially pressed (⬃100 MPa) from powdered, mudcracked pieces was placed in a ZAL-45AA fibrous alumina (Zircar Products, Inc., Florida, N.Y.) box within an ALC (Zircar) fibrous alumina cylinder. This assembly was positioned inside a quartz bell-jar atmosphere chamber of a Model 10 –1300 1300-watt benchtop microwave furnace (Microwave Materials Technologies, Inc., Knoxville, Tenn.). Temperature was sensed and controlled by a grounded and shielded Mo-sheathed Type C thermocouple in an alumina protective sleeve. After a series of evacuations with a direct drive pump and backfills with helium, the system was maintained in a helium atmosphere. Microwave power was applied to trigger ignition and self-propagating combustion of the sample, as noted from a bright flash and rapid heatup of the sensing thermocouple. Because the thermocouple tip was intentionally isolated from direct contact with the sample in order to protect the thermocouple, only a relative measure of the temperature during ignition could be obtained. After ignition and combustion, full micro-
1490
R.H. Plovnick, E.A. Richards / Materials Research Bulletin 36 (2001) 1487–1493
Fig. 2. Higher-magnification SEM photograph of microwave-combusted mudcracked piece.
wave power was applied for an additional 20 min. The system was then allowed to cool to room temperature in helium. 2.4. Analysis of reaction products Combusted samples were examined by X-ray diffraction analysis to identify crystalline phases. Data were collected using a Philips vertical diffractometer, copper K␣ radiation, and proportional detector registry of the scattered radiation. The level of each phase was estimated by use of relative intensity values determined from the strongest diffraction peak of each phase. An attempt was made to determine the average crystallite size from the X-ray diffraction analytical data, i.e. the peak intensities at half-width. Scanning electron microscopic (SEM) analysis was carried out on broken pieces of combusted mudcracked pieces and uniaxially pressed pellets.
3. Results and discussion Microwave ignition of the TiO2/2Al2O3䡠B2O3/Al samples occurred at temperatures in the neighborhood of 700⫺750°C, as estimated from the control/reference thermocouple. As noted above, temperature could only be estimated as a result of protective insulation surrounding the thermocouple. Maximum thermocouple temperatures observed approached
R.H. Plovnick, E.A. Richards / Materials Research Bulletin 36 (2001) 1487–1493
1491
Fig. 3. Lower-magnification SEM photograph of microwave-combusted uniaxially pressed pellet.
1475°C; again, these are only approximations of the actual temperature at the sample surface. It was not possible to take optical pyrometer readings with the available microwave furnace and sample containment system. The microwave-combusted pieces and pellets maintained their shape and weight, did not stick to one another, and were grayish-black vs. whitish-gray prior to combustion. The individual pieces and pellets had good qualitative cohesive strength and sufficient hardness to easily scratch glass deeply. Qualitative testing with a milliohmeter probe indicated the pieces and pellets to have low resistivity, with some surface contact resistance. The microwave-combusted pellets generally cracked into 2–3 pieces with relatively smooth surfaces. Under microscope at 40X, the interior exposed surfaces of cracked pellets appeared crystalline with fine porosity. X-ray diffraction analysis of microwave-combusted pieces (two runs) and pellets (three runs) indicated only ␣-Al2O3 and TiB2. The relative amount of each phase present as estimated by use of relative intensity values determined from the strongest diffraction peak of each phase was ⬃100 Al2O3: 60 TiB2. The attempt to determine the average crystallite size from the peak intensities at half-width showed that the crystallite size of both Al2O3 and TiB2 were ⬎1500 Å, the upper limit of crystallite size which could be determined by this method. Representative SEM analytical photographs are shown in Figures 1– 4 for a combusted mudcracked piece and uniaxially pressed pellet. There are some regions which appear relatively dense, especially in the pellets. The residual porosity is nonuniform, as would be expected from the preparative method used. This investigation has uncovered a new aluminothermic combustion synthesis route to
1492
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Fig. 4. Higher-magnification SEM photograph of microwave-combusted uniaxially pressed pellet.
TiB2-Al2O3 composite, using the reactants 2Al2O3䡠B2O3 (Al4B2O9), TiO2, and Al. Microwave-driven combustion was used for convenience, as it is known to be a useful technique for controllably igniting laboratory-scale aluminothermic reactions [7]. It should, however, be possible to duplicate the synthesis with other known methods of triggering combustion, e.g. resistance heating [8] and many of the specialized methods reviewed by Wang et al. [9] including electrical current, laser beam, and mechanical impact. A major limitation of combustion synthesis is that relatively high levels of porosity, up to 50%, are retained in the product [10]. The SEM photographs in Figs. 1– 4 show this to be the case for the present reaction scheme. In applications where dense final bodies are needed, the products from the present reactant scheme lend themselves to crushing to fine powder followed by a subsequent hot-pressing step. Alternatively, it may be possible to run the combustion synthesis under high pressure, as has been reported for other exothermic chemical reactions [11]. The fact that 2Al2O3䡠B2O3 is not moisture-sensitive or watersoluble, unlike boron-containing oxide reactants used previously to synthesize TiB2-Al2O3, could make use of 2Al2O3䡠B2O3 of particular advantage in high pressure synthesis and in more conventional approaches.
Acknowledgments We thank Brian T. Lynch for the X-ray diffraction analyses, and Chris J. Goodbrake for the scanning electron microscopic analyses.
R.H. Plovnick, E.A. Richards / Materials Research Bulletin 36 (2001) 1487–1493
1493
References [1] D.A. Hoke, D.K. Kim, J.C. LaSalvia, M.A. Meyers, Combustion synthesis/dynamic densification of a TiB2-SiC composite, J. Am. Ceram. Soc. 79 (1) (1996) 177–182. [2] A. Chrysanthou, D. Cheung, A. Jha, Self-propagating high temperature synthesis of Al2O3-TiB2 ceramics, Conf. on Ceramics - Charting the Future, Proc. 8th CIMTEC, Florence, 28 June– 4 July 1994 Techna Srl, 1973–1980 (1995). [3] K.V. Logan, J.D. Walton, TiB2 formation using thermite ignition, Ceram. Eng. Sci. Proc. 5 (7– 8) (1984) 712–738. [4] P.J.M. Gielisse, W.R. Foster, The system Al2O3-B2O3, Nature 195 (1962) 69 –70. [5] T. Kitamura, K. Sakane, H. Wada, H. Hata, H. Kanbara, Process for preparing aluminum borate whiskers, U.S. Pat No. 4,925,641, 1990. [6] G.W. Warner, Aluminum carboxylates, in: Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed, Vol. 2, Wiley, New York, 1978, pp. 202–209. [7] I. Ahmad, R. Dalton, D. Clark, Unique application of microwave energy to the processing of ceramic materials, J. Microwave Power Electromagn. Energy 26 (3) (1991) 128 –138. [8] J.D. Walton, Jr., N.E. Poulos, Cermets from thermite reactions, J. Am. Ceram. Soc. 42 (1) (1959) 40 – 49. [9] L.L. Wang, Z.A. Munir, Y.M. Maximov, Review [of] thermite reactions: their utilization in the synthesis and processing of materials, J. Mater Sci. 28 (1993) 3693–3708. [10] H.J. Feng, J.J. Moore, D.G. Wirth, Combustion synthesis of dense ceramic-metal composites, Ceram. Trans. 38 (1993) 103–114. [11] R.M. Marin-Ayral, J.C. Tedenac, M. Bockowski, M.C. Dumez, SHS experiments under high pressure, Ann. Chim. Fr. 20 (1995) 169 –180.